SE456202B - REVERSIBLE BATTERY CELL WITH PRESSURE LOADED LITHIUM ANOD - Google Patents

Description

20 25 30 35 456 202 For example, a battery with free-standing (non-pressure-loaded) lithium electrodes has a maximum of between about 1.6 - 2.5 conversion cycles when using electrolytes, which consist of 1 M LiAsF5 or 1 M LiClO4 in propylene carbonate. It would be highly desirable to be able to increase the reversibility of such electrodes and batteries.

It has been found that in accordance with the present invention, a significant increase in the number of conversion processes can be achieved for electrodes which form porous, externally amalgamated coatings thereon.

According to the present invention there is provided a reversible battery cell which is characterized by a cathode and an anode comprising a lithium metal substrate, an anhydrous electrolyte and a device for continuously applying a pressure load of at least 340 kPa to the anode during both charging and discharging the battery cell, the pressure load preventing the formation of a porous coating of outer, irregularly oriented alloy grains when the lithium metal is coated on the lithium metal substrate of the anode, thereby providing a substantially non-porous coating structure, which gives the anode improved reversibility.

The present invention also provides a method of manufacturing a battery and a method of using a battery to increase its reversibility by increasing the number of possible conversion processes within an electrode of the cell. The method of manufacturing the battery comprises the steps of constructing an electrolytic cell with a cathode, an anode and an electrolyte, said anode forming such a porous, external, amalgamated coating on the electrode. A compressive load is applied to such an anode as described above. As mentioned above, the electrode is preferably an alkali metal anode, for example a lithium anode, and the pressure load is preferably applied to the electrode continuously during both discharge and charging.

Electrode apparatus, battery and method according to the present invention offer a number of specific advantages. Applying the compressive load causes the particles or grains of the amalgamated coating on the electrode to be brought closer together. As discussed in more detail below, this can also lead to a reduction in the electrical resistance between the grains and lead to an increased resistance to metal ion migration through the porous coating from the grains in question. Thus, the present invention facilitates erosion of metal from the outer surface of the electrode (ie, from the front surface of the coating).

In an embodiment according to the invention, the electrolytic cell (battery) comprises at least one cathode, an alkali metal anode, at least one separating means arranged between anode and cathode, a non-aqueous electrolyte and means for applying a compressive load which exceeds the compressive strength of the amalamated the coating on the anode, i.e. the compressive load should be such that it deforms the coating to bring the grains in the coating closer together and reduce the porosity of the coating and reduce the electrical resistance between the grains in the coating. Preferably, the load exceeds the compressive strength of the substrate coated with the coating, i.e. the compressive load is large enough to physically deform the substrate. As stated above, this facilitates erosion of alkali metal from the electrolytic alkali metal grains at the front of the amalgamated coating (between the anode and the separating layer), with the result that the reversibility of the battery is considerably increased. Methods of manufacturing the battery of the present invention are also described.

The present invention provides particularly advantageous results with lithium electrodes. At a critical pressure over which the lithium electrode is deformed, a morphology of the coating is obtained which is drastically different from that formed at low pressure. The coatings obtained in the coating with lithium at low pressure are of a very porous nature, as evidenced by examination under an electron microscope, the grains being in the form of loose scales or thin bonded ski-like grains.

The coating coating obtained over the critical pressure is of a substantially non-porous nature. The grains are of regular pillar shape with their axes aligned perpendicular to the surface of the substrate.

The pillars are tightly packed with respect to each other so that the ends of the pillars form a non-porous, smooth surface parallel to the surface of the substrate. This type of coating can be maintained for many successive dissolution and coating cycles (discharge and charge). In special cases, where the pressure varies across the lithium electrode, it has been found that a sharp boundary exists between the porous coating type and the even, columnar type of coating. This shows that the coating morphology is strongly dependent on the pressure in the vicinity of the critical pressure.

According to another preferred embodiment, the cathode is of a type which results in an even current density, for example a MoS2 cathode, and the anode consists of an alkali metal, which internally has an alkali metal substrate and an outer amalgamated coating comprising electrolytic alkali metal alloy grains. coating layers (preferably formed by re-coating alkali metal on the anode). In a preferred embodiment of this kind, the cathode is a transition metal calcogenide, containing LiXMoS2 and the anode is lithium. Preferably, the LiXMoS2 cathodic active material is pretreated to operate in "Phase II" as described in US 4,224,390, the disclosure of which is hereby incorporated by reference.

For a full understanding of the invention, it will be described in the following with reference to the accompanying drawings, in which Fig. 1 schematically shows a battery in accordance with the invention, Fig. 2 schematically shows a spiral battery in accordance with the invention, zu 10 15 20 25 30 35 5 456 202 Fig. 3 schematically illustrates the winding operation of the spiral battery.

Some electrode materials such as alkali metals, such as lithium, are thermodynamically unstable in the presence of metal ion conducting electrolytes, which are in liquid form at ambient temperature. Aqueous electrolytes, for example, react violently with alkali metals to form alkali hydroxides and hydrogen gas. This reaction is often so violent that it can be said to be explosive. However, some electrolytes react less violently with electrode metals to form kinetically stable passivation films on the surface of the metal electrode. These latter electrolytes can be used for the construction of practical cells where metal electrodes are used.

After cycling through with such a metallic electrolytic cell, for example, two parts of the electrode become physically isolable. These parts consist of (1) a central, substantially non-porous metal substrate with a passivation film and (2) a porous, coated, amalgamated coating of electrolytically active metal grains, each grain having a passivation film.

As soon as such a metal electrode is exposed to the action of an electrolyte, a chemical reaction will begin to occur.

The reaction of the electrolyte with the metal produces a passivation film on the surface of the metal. This passivation film is essentially non-porous, yet ion permeable. The film tends to isolate the metal grains electrochemically. The desired electrical conductivity of the film on the grains balances between an increase in the degree of passivation reaction due to too high conductivity and a decrease in the electrochemical activity of the grains due to too low conductivity. While a low conductivity reduces the reactivity of electrolyte and metal, the low conductivity erosion of metal from the substrate instead of from the inside of the grains increases (due to the high contact resistance between the grains).

In order to achieve a high number of reaction processes (T) and to minimize the surface area of the metal electrode (so that the reaction with the electrolyte to form further passivated metal is minimized), it is advantageous that erosion of the electrode lytically active metal preferably occurs on the front (outside) of the coating rather than inside it or at the surface of the underlying non-porous substrate. If sealing does not take place on the front (outside) while the underlying parts of the substrate are being sealed, the front loses physical contact with the rest of the coating and the substrate. As a result, the front becomes electrochemically inactive. Compression load of the electrode over the compressive strength of the coating (ie to deform the coating in order to bring the grains together in the coating) makes it possible to preferably wipe off the front. I Three factors can contribute to the resistance to erosion of the various parts of the electrode during battery use.

These resistance factors are: (l) the electrical resistance between the grain in question (of the coating) and the current collector; (2) the ionic resistance associated with the migration of metal ions through the porous coating of the grains in question; and '(3) the resistance associated with the erosion of a metal ion from a grain and the transport of the ion through a passivating film.

With respect to factor (1), the electrical resistance is normally highest for the grains closest to the front of the coating. In fact, it is reasonable to assume that the electrical resistance is substantially zero for grains lying at the substrate: surface. With respect to factor (2), the ionic resistance is greatest for the substrate and decreases for grains that are closer to the front of the coating. The ionic resistance is lowest at the front of the coating where the diffusion path for ions to reach active grains is shortest, and is greatest at the substrate, to which the diffusion path is longest. Finally, with respect to the factor (3), the counter of the passivation film is controlled 10 15 20 25 30 35 7. 456 202 state of the chemical nature of the passivation film and cannot be substantially changed by changing the physical parameters of the coating. By applying a compressive load to the surface of the amalamated coating (preferably perpendicular to the coating), which compressive load exceeds the compressive strength of the coating as explained above, a double effect is obtained. First, the porosity of the coating is reduced by moving the grains closer to each other when the coating is compressed. The decrease in porosity results in an increase in the ion resistance to erosion to a greater degree for the substrate than is the case for the front. At the same time, the electrical resistance between the grains of the coating is reduced, as the contact area between adjacent grains increases. The net result of the compression is thus an increase in the sum of the three resistance factors in the vicinity of the substrate and a decrease in the sum of these resistance factors for grains located near the front of the coating, whereby the desired effect of an increase in the reversibility of the battery achieved. (The front is also conveniently identified as the interface between the electrode and S @ PäY2fï fi9 S0F9ö fi @ t).

The present invention can be used with any battery utilizing an electrode which is to react with the electrolyte to form an amalgamated, porous coating on the electrode, particularly during charging. For example, anode materials such as alkali metals, alkaline earth metals and transition metals such as zinc form coatings on the metal by reaction with certain electrolytes. Thus, for example, alkali metals form lithium in the presence of a non-aqueous electrolyte such as propylene carbonate comprising LiClO

The compressive load is, as explained above, § 15 ß W such that it will deform the coating by compressing particles or grains of the coating so that they are brought closer together. Accordingly, the compressive load to be used according to the present invention varies depending on the nature of the electrode, electrolyte and coating. A softer metal requires a lower compressive load. For example, the compressive load under which alkali metals are deformed is typically low, and all alkali metals are soft and malleable, for example, the tensile strength of lithium is of the order of 60-80 psi.

In view of the fact that the coating is a porous metal coating, in which the cavities are filled with liquid electrolyte, the compressive strength (ie the force at which the material is deformed under pressure) is less or equal to that of pure metal.

The pressure load does not necessarily have to be applied continuously during charging and discharging. Application of compressive load to compress coating can in fact be of short duration, for example by applying a compressive load for a period of time at the end of the charging cycle or even applying the compressive load after charging and before further use. However, the compressive load is preferably applied continuously at least during charging.

In the case of lithium, the pressure load is preferably applied from about 50 to about 500 psi continuously during the charging process. As mentioned above, such a pressure load on the lithium electrode during charging (eg lithium with a suitable substrate) results in the electrode being coated with material grains in column form with the shafts substantially perpendicular to the substrate.

Applying a pressure load to the electrode loads the materials from which the entire cell is constructed. The components of the cell are preferably soft and flexible, so that the load can be applied evenly distributed. The use of expanded metal gratings as current collectors and hard, granular powders as electroactive materials is not advisable. The material of the Separator should also be flexible. Suitably metallic foils are used as current collectors and soft materials such as graphite or molybdenum sulfide (transition metals of caikagenia catadam type) are used in the cathode. If possible, the cathode is intended to have an even current density to ensure uniform utilization of the substrate.

Polypropylene or other suitable flexible, porous or semi-permeable separating means are preferred.

As shown in Fig. 1, the means used to apply the compressive load may consist of a simple coil spring 10, which is supported by a compression plate 12 arranged at the top of the battery. Of course, other suitable pressure means can be used. Fig. 2 shows a coil battery in which an elastic separating means and a C-clanuna 10a are sufficient to achieve the desired reduction of the porosity in the coating and the desired reduction with respect to electrical resistance between the grains in the coating.

As further shown in Fig. 1, the electrolytic cell (battery) comprises an anode 14 (with a corresponding pantograph) arranged in layers between two cathodes 16 (with a corresponding pantograph). Electrolyte-saturated separating means 18 insulate the ano 14 from the cathodes 16 and are carriers of the electrolyte intended for the cell in their pores. The anode, the cathodes and the separating means form a sequence which is electrochemically active to generate current. The anode has such a composition; that a porous, amalgamated coating is formed thereon as discussed above. Placed in a housing 20, the cell is subjected to compression as already described. The housing 20 is preferably hermetically sealed in a non-reactive. atmosphere.

As illustrated in Fig. 3, in the manufacture of a spiral cell (battery), the elasticity of two separation layers 18-18, one between the anode 14 and the cathode 16 and the other on the outside, is used to provide a radial compressive load on the intended electrode, i.e. either the anode 14 or the cathode 16, by winding the bearings tightly in a loop around a conductor. The compressive stress on the separation layers is maintained by means of the C-clamp 10a to generate the desired compressive load. Polypropylene can be used in layers 18-18. The following examples are intended to illustrate the electrode apparatus, battery and method of the present invention and are not to be construed as limiting the scope of the invention.

Example 1 An electrolytic cell is produced between two flat, rigid pressure plates. The cathode consisted of a surface-treated molybdenite powder, which was evenly distributed on an aluminum foil substrate, as described in US 4,251,606. The cathode provides an even current density of the cell. The molybdenite powder was distributed at 10 mg / cm 2 on the aluminum foil. The area of the cathode was 5.6 cm 2. The anode consisted of a sheet of lithium foil of corresponding dimensions and having a thickness of about 125 microns and was layered between two cathodes by means of microporous separating means of polypropylene (Celgard 2500 available from Celanese Corporation). The electrolyte consisted of 1 M LiAsF6 in propylene carbonate. The propylene carbonate was first subjected to a purification to a total degree of contamination of less than about 100 ppm. The cathode and separators were first saturated with electrolyte.

The cell was assembled between printing plates, the cell being subjected to a pressure of 1.9 kp / cm 2 (27 psi) by means of the plates.

The complete cell was enclosed in a hermetically sealed container filled with argon gas. A glass / metal seal was used for the current lead-through for the negative connection of the electrolytic cell. The cell was assembled to convert the cathodic material to "Phase II" LixMoS2, as described in US 4,224,390. Measures had been taken to ensure that the electrolyte did not deteriorate during the conversion process. repeatedly at a current of 2 mA both for charging and discharging and between a lower voltage limit of 1.3 volts for discharging and an upper limit of 2.6 volts for charging. decreased to 50% of the charging capacity measured at the end of the tenth cycle. - by taking the ratio of this charge amount compared to the theoretically expected amount of charge if the whole had been discharged in a process cycle, the number of turnover ratios was opp (T) for the lithium anode equal to three.

Example 2 An electrolytic cell constructed in the same manner as that of Example 1 in all respects except that the electrodes were subjected to a pressure of 3.5 kp / cm 2 (50 psi), was subjected to process cycles under conditions identical to those of Fig. 1 described. The number of conversion cycles (T) for the lithium anode in this second Cell was equal to eight.

Example 3 An electrolytic cell of the same design as that of Example 1 in all respects except that the electrodes were subjected to a pressure of 7 kp / cm 2 (100 psi) was subjected to process cycles under conditions identical to those described in Example 1. The number of reactions (T) for the lithium anode in this third cell was equal to nine.

Example 4 An electrolytic cell which was identical in all details to the cell of Example 1 except that the electrodes were subjected to a pressure of 12 kp / cm 2 (170 psi) was subjected to process cycles under conditions identical to those described in Example 1. Example 1. The number of conversion cycles (T) of the lithium anode in this fourth cell was equal to eleven.

Example 5 An electrolytic cell which was similar in all respects to the cell of Example 4 except that the support electrolyte used consisted of 0.5 M LiClO 4 instead of 1 M LiAsF 6, was constructed and tested under the same conditions as those set forth in Example 3. The number of sales processes (T) was equal to seven. Example 5 shows that the application of pressure played an at least as significant role in determining the number of conversion processes as the electrolyte in the cell. Although the number of turnover rates varies with the choice of electrolyte, the number of turnover rates that can be achieved by applying pressure to the cell is always greater than the number of turnover rates that can be achieved when the cell operates independently. få m!

Claims (10)

10 15 20 25 30 35 456 202 13 PATENT REQUIREMENTS 1. l. Reversible battery cell, characterized by a cathode (16) and an anode (14), comprising a lithium metal substrate, an anhydrous electrolyte and a device (10, 10a) for continuous application of a a pressure load of at least 340 kPa on the anode during both charging and discharging of the battery cell, the pressure load preventing the formation of a porous coating of outer, irregularly oriented alloy grains when the lithium metal is coated on the lithium metal substrate of the anode, thereby providing a porous building, which gives the anode improved reversibility. A reversible battery cell according to claim 1, characterized in that the pressure load is from 340 to 3400 kPa. A reversible battery cell according to claim 1 or 2, characterized in that the coated lithium metal comprises tightly packed grains with pillars, the axes of which are aligned substantially perpendicular to the substrate. A reversible battery cell according to claims 1-3, characterized by a separator (18) located between the anode (14) and the cathode (16). Reversible battery cell according to one of the preceding claims, characterized in that the cathode provides an even current density for the battery cell. Reversible battery cell according to one of the preceding claims, characterized in that the cathode (16) is a calcogenic cathode of a transition element metal containing LiXMoS2. A reversible battery cell according to claim 6, characterized in that the cathode comprises MoS2 as a cathode active material. Reversible battery according to claim 7, characterized in that the cathodic active material comprises Mo52 as a surface treated with Mo02. A reversible battery according to claim 4, characterized in that the cathode, anode and separator are substantially planar, the pressure load being applied by combing these elements into melan pïatta pressure plates. Reverse battery according to claim 4, characterized in that the cathode, anode and separator are tightly wound in the form of a spiral. Mr.